I have a situation where tcpdump (libpcap) sees TCP SYNs ... but a sniffer
located on the wire just outside the NIC does not see those TCP SYNs
Any tips on how to follow a frame through netdev.c to figure out where it
gets dropped?
>From http://lxr.linux.no/#linux+v3.8/net/core/dev.c
[...]
2361 <http://lxr.linux.no/linux+*/net/core/dev.c#L2361>
if (!list_empty
<http://lxr.linux.no/linux+*/+code=list_empty>(&ptype_all
<http://lxr.linux.no/linux+*/+code=ptype_all>))2362
<http://lxr.linux.no/linux+*/net/core/dev.c#L2362>
dev_queue_xmit_nit
<http://lxr.linux.no/linux+*/+code=dev_queue_xmit_nit>(skb
<http://lxr.linux.no/linux+*/+code=skb>, dev
<http://lxr.linux.no/linux+*/+code=dev>);2363
<http://lxr.linux.no/linux+*/net/core/dev.c#L2363>2364
<http://lxr.linux.no/linux+*/net/core/dev.c#L2364>
skb_len <http://lxr.linux.no/linux+*/+code=skb_len> = skb
<http://lxr.linux.no/linux+*/+code=skb>->len
<http://lxr.linux.no/linux+*/+code=len>;2365
<http://lxr.linux.no/linux+*/net/core/dev.c#L2365> rc
<http://lxr.linux.no/linux+*/+code=rc> = ops
<http://lxr.linux.no/linux+*/+code=ops>->ndo_start_xmit
<http://lxr.linux.no/linux+*/+code=ndo_start_xmit>(skb
<http://lxr.linux.no/linux+*/+code=skb>, dev
<http://lxr.linux.no/linux+*/+code=dev>);2366
<http://lxr.linux.no/linux+*/net/core/dev.c#L2366>
trace_net_dev_xmit
<http://lxr.linux.no/linux+*/+code=trace_net_dev_xmit>(skb
<http://lxr.linux.no/linux+*/+code=skb>, rc
<http://lxr.linux.no/linux+*/+code=rc>, dev
<http://lxr.linux.no/linux+*/+code=dev>, skb_len
<http://lxr.linux.no/linux+*/+code=skb_len>);2367
<http://lxr.linux.no/linux+*/net/core/dev.c#L2367> if
(rc <http://lxr.linux.no/linux+*/+code=rc> == NETDEV_TX_OK
<http://lxr.linux.no/linux+*/+code=NETDEV_TX_OK>)2368
<http://lxr.linux.no/linux+*/net/core/dev.c#L2368>
txq_trans_update
<http://lxr.linux.no/linux+*/+code=txq_trans_update>(txq
<http://lxr.linux.no/linux+*/+code=txq>);2369
<http://lxr.linux.no/linux+*/net/core/dev.c#L2369>
return rc <http://lxr.linux.no/linux+*/+code=rc>;2370
<http://lxr.linux.no/linux+*/net/core/dev.c#L2370> }2371
<http://lxr.linux.no/linux+*/net/core/dev.c#L2371>2372
<http://lxr.linux.no/linux+*/net/core/dev.c#L2372>gso
<http://lxr.linux.no/linux+*/+code=gso>:2373
<http://lxr.linux.no/linux+*/net/core/dev.c#L2373> do {
[...]
I believe that libpcap gets the frame somewhere inside dev_queue_xmit_nit()
... which suggests to me that the execution reaches Line 2362 just fine ...
which suggests that execution reaches Line 2365:
2365 <http://lxr.linux.no/linux+*/net/core/dev.c#L2365>
rc <http://lxr.linux.no/linux+*/+code=rc> = ops
<http://lxr.linux.no/linux+*/+code=ops>->ndo_start_xmit
<http://lxr.linux.no/linux+*/+code=ndo_start_xmit>(skb
<http://lxr.linux.no/linux+*/+code=skb>, dev
<http://lxr.linux.no/linux+*/+code=dev>);
and ndo_start_xmit() ends up calling e1000_xmit_frame ...which gets
me, I believe, to Line 6039 in e1000e-2.5.4\src\netdev.c
static netdev_tx_t e1000_xmit_frame(struct sk_buff *skb,
struct net_device *netdev)
{
struct e1000_adapter *adapter = netdev_priv(netdev);
struct e1000_ring *tx_ring = adapter->tx_ring;
unsigned int first;
unsigned int tx_flags = 0;
unsigned int len = skb_headlen(skb);
unsigned int nr_frags;
unsigned int mss;
int count = 0;
int tso;
unsigned int f;
if (test_bit(__E1000_DOWN, &adapter->state)) {
dev_kfree_skb_any(skb);
return NETDEV_TX_OK;
}
[...]
So, now I want to figure out which 'if' clause I hit for these particular
TCP SYNs .. I suppose I could add my own 'print' statements:
if (test_bit(__E1000_DOWN, &adapter->state)) {
dev_kfree_skb_any(skb);
printf ("Link down, discarding frame");
return NETDEV_TX_OK;
}
although ... I doubt that string will go anywhere useful ... I'd rather
send the string to syslog, and I don't know how to do that yet
Anyway, before I started walking this path, I figured I'd ask for tips.
How would you tackle this?
I'm running kernel 3.8.0-34 (Precise Pangolin / LTS 12.04.3); Intel 82579LM
(rev06)
[Interestingly enough, most TCP conversations pass across this NIC just
fine ... just one particular app (NFS) for which the TCP SYNs don't reach
the wire.]
--sk
Stuart Kendrick
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